1. Field of the Invention
This invention relates to the use of phototransistors employed for the detection of variable light signals. This invention can be employed in conjunction with the detection of visible light signals, although it is primarily intended for use in detecting infrared signals. The preferred embodiment of this invention provides a means for detecting varying infrared LED pulses while compensating for ambient conditions, such as excitation of the phototransistor by ambient light. Although this invention can be employed with single emitter--phototransistor pairs, it is especially adapted for use with systems having a large number of pairs. The preferred embodiment of this invention is suitable for use in systems in which the emitter--phototransistor pairs are scanned at rapid rates. Although this invention would be suitable for use in applications such as invisible light barriers used for safety around machinery or as anti-intrusion barriers, the invention is especially adapted for and is depicted in use with a touch entry system used to input data through a video display.
2. Description of the Prior Art
Coincident with the use of video displays has always been the problem of the man/machine interface. Traditionally, control of displayed information or cursors has been via a keyboard. Recently, however, a number of devices have been introduced which allow an operator to directly interact with the video display. These types of devices have included light pens, desk-type mouse controllers, or touch input devices such as a switch matrix or opto-electronic matrices. While generally switch-type overlays, placed adjacent a video display, are inexpensive to apply and utilize, they are generally susceptible to contact wear as well as distortion of the video information which is presented to the viewer, particularly in high usage environments. However, since opto-matrix schemes utilize light, which is generally in the infrared region, the switch matrix presented by the light beams is invisible to the viewer and, therefore, does not distort the video information displayed to the viewer and is not subject to wear in high usage environments. A number of schemes which utilize opto-matrix frames may be found in U.S. Pat. No. 4,267,443 "Photoelectric Input Apparatus", issued May 12, 1981 to Carroll et al; and U.S. Pat. No. 3,764,813 "Coordinate Detection System", issued Oct. 9, 1973 to Clement et al. These three schemes address problems inherent with opto-matrix devices such as increasing frame resolution without a corresponding increase in components, surrounding or ambient light compensation, or optimization of emitter/detector driving and detecting networks respectively. These systems still have drawbacks. Large component usage results in higher costs. Some systems exhibit difficulty in compensating for reflection or glare resulting in styli hits which are not recorded. Glare problems generally occur when an emitter is adjacent the corner of the frame or bezel such that light produced by the emitter bounces off an adjacent surface and is then detected by the detector. It follows that a stylus which is introduced to the video display may block the majority of the light received by the detector, but the amount of reflected light may be sufficiently great that the detector and its associated circuitry do not perceive a hit.
Ambient light also results in significant problems. Several systems for ambient light compensation have been proposed, and some are marketed in touch entry systems. For example, U.S. Pat. No. 4,267,443 discloses an ambient light sampling system. U.S. Pat. No. 3,970,846 discloses an ambient light compensation feedback loop for affecting the bias voltage at the base of a phototransistor. Commonly assigned U.S. patent application Ser. No. 621,585 filed June 18, 1984, and incorporated herein by reference, discloses a digital ambient light sampling approach. It is desirable to have a device which minimizes the number of components necessary for addressing and detecting emitters and detectors. It is also desirable that the device dynamically compensates for ambient light and for variations in emitter output and detector sensitivity. At the same time the device should employ a minimal number of components for addressing and detecting emitters and detectors. Such a scheme is taught in the present invention.
It is a principal object of the invention to provide a practical touch input device comprising a four-sided frame, optical emitters disposed in two adjacent sides of the frame, optical detectors disposed in the two sides of the frame opposite the emitters, and sampling device for sequentially sampling the light received by the detectors. The system should also include an ambient compensation network for maintaining the output level of ambient induced signals from phototransistors within the phototransistor operating range while permitting the signal detection gain to be sufficiently large to permit reliable and easy detection of a pulsed signal.
An object of this invention is to arrive at a cost effective compensation technique to neutralize the effects of ambient light in the phototransistor biasing network. Ambient light alters the operating level of the bias network and effectively limits the system dynamic range. The variance of this operating level, as a function of ambient light, limits the gain at which the phototransistor can be operated. With compensation, this gain can be increased significantly. Additionally, the dynamic operating range can be increased and a wider range of devices will be acceptable.